99 Suburban Misfire

I have a 99 Suburban with 5.7LV8. Bought it last year with 155k on it and ran great for almost a year. Started misfiring bad and discovered cat on passenger side blown out. Replaced it along with fuel filter, plugs, wires, cap, rotor and still having misfire. Have an OTC computer to scan codes and engine diagnostics. Current codes are P0300B, P0155, and P0101. While watching engine test with it running, always misfires on Cyl 4 and 6. Mass Air Flow is at 9 g/s at idle. Misfire is noticeable at all times, but smooths out some at highway speeds. Gas mileage is terrible and gas smell is very strong at idle in the driveway. Can test almost anything with the computer I have, but have nothing to tell me what is acceptable numbers. Any help I can get is greatly appreciated as this is my wife's daily driver!!
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Try cleaning the Mass airflow sensor if that doesn't work it might need to be replaced its probably causing the PO300 also,the PO155 is probably a bad heater in that O2 sensor i had three of them fail on different make cars last year unless theres a problem with the wiring going to it you have to replace it to fix that code

The voltage across terminals B and A should be 450 mV (0.45 V) and C and D, close to the battery voltage with key on, engine off. You may want to post the freeze frame data for each code so we have a better picture of what's going on.

A mechanic friend suggested I upgrade to the CBI for about $300. The fuel smell is very noticeable now even with the vehicle not running. He said I could tear into the existing injection set up, but if I found problems I would be ahead to upgrade regardless. Would this be a good investment even if this isn't my misfire problem? Thxs